The present invention relates to continuously transmitting and receiving radar, for example a continuous wave (CW) radar, and more particularly to the suppression of leakage (otherwise known as feedthrough) in such a radar.
A leakage or feedthrough signal comprises an unpropagated portion of the transmitted energy which is fed directly to the radar receiver resulting in saturation of, damage to and/or degradation of sensitivity of the receiver.
The Radar Handbook (Editor M. I. Skolnik, McGraw-Hill, New York, 1970), pages 16-18 and 16-19 discusses minimisation of feedthrough using a dynamic canceller. All dynamic cancellers depend on synthesising a proper amplitude and phase of a signal at microwave frequencies taken from the transmitter and using this to cancel the feedthrough signal. These pages also mention that microwave feedthrough cancellation is of principal value in preventing saturation and in minimising the effects of AM noise. Because of the correlation effect, FM noise produced by feedthrough tends to cancel in the receiver. Near-in AM and FM noise produced by clutter is also beneficially reduced by a feedthrough servo, since, in nulling out the carrier, it automatically removes both sidebands, whatever their origin, as long as the decorrelation interval is short. Clutter signals from long ranges have both AM and FM noise that is essentially decorrelated, and feed-through nulling of these signals may increase their deviation by a factor of 2 or their power by a factor of 4.
A method of cancelling the effects of noise sidebands present in the leakage signal is disclosed in British Patent Specification No. 2147473B which describes a method of FM noise reduction in a CW radar system. The system comprises a master oscillator, means for transmitting an RF signal derived from the oscillator and for receiving a return signal, and a mixer having a local oscillator port and a signal port. A local oscillator signal is obtained by coupling-out a portion of the master oscillator signal. A leakage signal may reach the signal port of the mixer otherwise than along the propagation path of the local oscillator signal and without being reflected externally of the system, for example by reflection of the signal supplied to the transmitting means. In order to minimise noise in the mixer due to FM noise in the leakage signal, the electrical lengths over the operating frequency range of the system from the master oscillator to the local oscillator port and to the signal port of the mixer of the propagation paths of the local oscillator signal and the leakage signal respectively are made substantially equal, for example by including a delay line in the local oscillator signal path. Since FM noise power decreases with increasing offset from the carrier frequency (that is the transmitted frequency), this known method of noise reduction is particularly suited to alleviating noise at low intermediate frequencies (IFs). Additionally noise reduction systems are particularly useful with frequencies in the millimeter-wave range for which currently available oscillators tend to be rather noisy.